text: Primary production in the ocean is heavily dependent on the presence of nutrients and the availability of sunlight. Here, nutrients refers to nitrogen, nitrate, phosphate, and silicate, all important nutrients in biogeochemical processes that take place in the ocean. A commonly accepted method for relating different nutrient availabilities to each other in order to describe chemical processes is the Redfield, Ketchum, and Richards (RKR) equation. This equation describes the process of photosynthesis and respiration and the ratios of the nutrients involved.
  The RKR Equation for Photosynthesis and Respiration:
  With the correct ratios of nutrients on the left side of the RKR equation and sunlight, photosynthesis takes place to produce plankton (primary production) and oxygen. Typically, the limiting nutrients to production are nitrogen and phosphorus with nitrogen being the most limiting.
  Lack of nutrients in the surface waters of subtropical gyres is related to the strong downwelling and sinking of particles that occurs in these areas as mentioned earlier. However, nutrients are still present in these gyres. These nutrients can come from not only vertical transport, but also lateral transport across gyre fronts. This lateral transport helps make up for the large loss of nutrients due to downwelling and particle sinking. However, the major source of nitrate in the nitrate-limited subtropical gyres is a result of biological, not physical, factors. Nitrogen in subtropical gyres is produced primarily by nitrogen-fixing bacteria, which are common throughout most of the oligotrophic waters of subtropical gyres. These bacteria transform atmospheric nitrogen into bioavailable forms.